This invention relates to an atomic and molecular beam detector that is universal in the sense that it is useful with all particles that have mass including those that do not readily ionize.
Various types of detectors are used in the prior art to detect the presence of atomic and molecular beams. A large number of these detectors require ionization of the beam particles. In one form of ionization detector known as the surface-ionization detector, the atom or molecule is caused to strike a heated wire thereby causing the particle to be ionized. Once the particle is ionized, it can be measured in any one of several different ways including current measurement with a sensitive electrometer, current measurement with the aid of an electron multiplier tube and electronic counting with an electron multiplier tube. The surface ionization detector is useful, however, only with easily ionizable elements, such as the alkalai metals, and is not useful in detecting the presence of beams that are composed of molecules and atoms of hydrogen, helium and other type elements that cannot be easily ionized.
Still another form of ionization detector is known to the art as an electron-bombardment ionizer. In this type of detector a high energy electron beam is directed at the element or molecule to be detected and the resulting ionized particle is detected and identified by means of a mass spectrum analyzer. This form of ionization detector is much more versatile than the surface-ionization detector, but it is also much more complicated, more expensive, and not as sensitive.
One type of detector used in the prior art to detect beams that cannot be surface ionized is the Stern-Pirani detector. In this detector, the beam is caused to enter a channel that leads to a small volume chamber containing a hot wire. The channel is ordinarily long and narrow to increase the outward flow resistance of the gas in the chamber so a higher equilibrium pressure can be achieved. Several chambers are utilized and the wires in these chambers are usually arranged in a bridge circuit in order to detect small changes in the resistance of the hot wire. The cooling of the wire in each chamber arises primarily from gaseous conduction and thermal radiation. The gaseous conduction, in turn, is dependent on the pressure in the chamber. By admitting the beam to one of the chambers, the pressure in this chamber is increased thereby causing a small change in the resistance of the corresponding wire which can be detected as an indication that the beam is present. This Stern-Pirani type detector requires careful temperature control and a rather precision block containing the chambers. A much simpler apparatus would be desirable to detect the presence of all types of atomic and molecular beams.